Known power seat slide apparatuses described, for example, in JP11-334424A and JP2004-345541A includes a lower rail fixedly mounted on a floor of a vehicle, an upper rail held by the lower rail so as to slide in a vehicle's longitudinal direction and supporting a seat for a vehicle, a screw shaft provided between the lower rail and the upper rail and extending in a direction of the upper rail's sliding motion, a nut member fixedly mounted on a lower rail and threadably engaged with the screw shaft, a gear box held on an upper rail and connected to an end of the screw shaft and a motor transmitting a drive force to the screw shaft via the gear box, where the screw shaft is rotated by means of motor rotation so that the upper rail slides relative to the lower rail in a longitudinal direction.
The known power seat slide apparatus described in JP 1-334424A includes a worm wheel rotatably supported in a casing of reduction gears and meshed with a worm driven by a drive motor, the worm wheel being integrally connected with an end of the screw shaft by a screw and nut arrangement.
The known power seat slide apparatus described in JP2004-345541A includes a screw member held by an upper rail via a gear box and the screw member's movement, relative to the gear box, in a sliding motion direction of the upper rail is restricted.
However, the power seat slide apparatus described in JP11-334424A includes a structure where the worm wheel and the screw shaft are rigidly fixed and where the screw shaft is supported by the worm wheel and a nut member, and it is therefore difficult to securely support the screw shaft of long length, which attributes to interaction between an engagement of the worm wheel and the worm gear, and an engagement of the screw shaft and the nut member. This may cause irregular sliding motion and/or irregular rotation of the screw shaft, leading to drawbacks, such as for example, a noise attributed to an engagement that is likely to occur when a seat is driven by the motor to slide.
Also, in the power seat slide apparatus described in JP2004-345541A, a load which works on the upper rail in a vehicle's longitudinal direction when a vehicle suddenly stops, is transmitted to the lower rail via the gear box, the screw member, and the nut member in the mentioned order. In other words, the load applied to the upper rail is inputted to the gearbox. Consequently, with the construction of the power seat slide apparatus described in JP2004-345541A, simplification of the structure and of the configuration related to the gear box, namely a size of the gear box, a holding structure of the gear box to the upper rail, and so forth, is difficult because the gear box is required to have a robust structure to withstand the load.
A need thus exists for a power seat slide apparatus which is not susceptible to the drawback mentioned above.